[PATCH 2 of 4 V3] manifest: use a size 3 LRU cache to store parsed manifests

[Siddharth Agarwal]

# HG changeset patch
# User Siddharth Agarwal <sid0 at fb.com>
# Date 1360424582 0
# Node ID 6cab05663f2d0ade01267e103cae12634fd49c28
# Parent  c5e8d2d68a8e1a7bada44d98cbe528f8b25dae15
manifest: use a size 3 LRU cache to store parsed manifests

Previously, the manifest cache would store the last manifest parsed. We could
run into situations with operations like update where we would try parsing the
manifest for a revision r1, then r2, then r1 again. This increases the cache
size to 3 to avoid that bit of performance fragility.

diff --git a/mercurial/manifest.py b/mercurial/manifest.py
--- a/mercurial/manifest.py
+++ b/mercurial/manifest.py
@@ -28,7 +28,8 @@ class manifestdict(dict):
 
 class manifest(revlog.revlog):
     def __init__(self, opener):
-        self._mancache = None
+        # we expect to deal with not more than three revs at a time in merge
+        self._mancache = util.lrucachedict(3)
         revlog.revlog.__init__(self, opener, "00manifest.i")
 
     def parse(self, lines):
@@ -51,12 +52,12 @@ class manifest(revlog.revlog):
     def read(self, node):
         if node == revlog.nullid:
             return manifestdict() # don't upset local cache
-        if self._mancache and self._mancache[0] == node:
-            return self._mancache[1]
+        if node in self._mancache:
+            return self._mancache[node][0]
         text = self.revision(node)
         arraytext = array.array('c', text)
         mapping = self.parse(text)
-        self._mancache = (node, mapping, arraytext)
+        self._mancache[node] = (mapping, arraytext)
         return mapping
 
     def _search(self, m, s, lo=0, hi=None):
@@ -102,8 +103,9 @@ class manifest(revlog.revlog):
     def find(self, node, f):
         '''look up entry for a single file efficiently.
         return (node, flags) pair if found, (None, None) if not.'''
-        if self._mancache and self._mancache[0] == node:
-            return self._mancache[1].get(f), self._mancache[1].flags(f)
+        if node in self._mancache:
+            mapping = self._mancache[node][0]
+            return mapping.get(f), mapping.flags(f)
         text = self.revision(node)
         start, end = self._search(text, f)
         if start == end:
@@ -143,7 +145,7 @@ class manifest(revlog.revlog):
 
         # if we're using the cache, make sure it is valid and
         # parented by the same node we're diffing against
-        if not (changed and self._mancache and p1 and self._mancache[0] == p1):
+        if not (changed and p1 and (p1 in self._mancache)):
             files = sorted(map)
             checkforbidden(files)
 
@@ -156,7 +158,7 @@ class manifest(revlog.revlog):
             cachedelta = None
         else:
             added, removed = changed
-            addlist = self._mancache[2]
+            addlist = self._mancache[p1][1]
 
             checkforbidden(added)
             # combine the changed lists into one list for sorting
@@ -208,6 +210,6 @@ class manifest(revlog.revlog):
             text = util.buffer(arraytext)
 
         n = self.addrevision(text, transaction, link, p1, p2, cachedelta)
-        self._mancache = (n, map, arraytext)
+        self._mancache[n] = (map, arraytext)
 
         return n